Method of acidizing wells



rect-ly above and rests on a packer.

United States Patent 3,236,305 METHGD 0F AClDlZlNG WELLS Christ F. Parks, Tulsa, Okla, assignor to The Dow Chemical Company, Midland, Mich. a corporation of Delaware N0 Drawing. Filed May 27, 1963, Ser. No. 283,594 4 Claims. (Cl. 166-42) This application is a continuation-in-part of application Serial No. 82,677, now abandoned filed January 16, 1961.

The invention relates to an improved method employing an aqueous composition as a Workover fluid for reworking or treating a well which has been or is a producing well.

The term, aqueous composition, as used herein, includes solutions, dispersions, slurries, gels, and emulsions in which water is employed as the liquid medium. Aqueous compositions may refer to fresh water, brines, oilwater emulsions, gum-thickened water, soapthickened gels, and the like with or without organic or inorganic materials dissolved, suspended, or slurried therein.

The term, Workover fluid, refers herein to fluids used primarily in reworking, e.g., altering, repairing, or cleaning a well, to perforating casing, to holding packers, in place, to fracturing, acidizing, and plugging off sand and water. For a discussion of Workover fluids and their use, reference is recommended to a Reader Service Reprint of The Petroleum Engineer, Drilling and Producing, (1956), entitled Well Workover Handbook, available from The Petroleum Engineer Publishing Company, PO. 1589, Dallas 21, Texas. For example, pages 6 to 11, thereof, sets out and discusses Workover operations. The term, Workover fluid, and reworking a well, are used herein in the sense employed in the discussion set forth in the cited publication.

One embodiment of the invention relates especially to such method wherein the use of a Workover fluid of increased density is required, as a packer fluid, i.e., a fluid to hold a packer in position which advantageously is a high density aqueous liquid which is positioned di- Such fluids to be fully satisfactory should be stable to resist degeneration by heat for relatively long periods of time, and should possess low fluid loss properties since they sometimes are retained in a well, e.g., in the annulus between casing and tubing, for prolonged periods of time. Fluids used in such Workover treatments as repairing or perforating casing are often used without packers, being preceded and/ or followed by drilling mud (as a convenient and economical weighing material) which must be kept from contaminating the level of treatment. It is highly desirable that such workover fluid be relatively viscous to maintain good separation at the contacting faces of the mud.

Patented Feb. 22, 1966 Another embodiment of the invention relates especially to an improved method of acidizing or combined fracturing and acidizing fluid-containing subterranean formations penetrated by a well. An aqueous acidizing composition always contains a substance reactive with the formation for the purpose of enlarging passages therethrough and thereby increasing the rate and amount of production of formation fluids therefrom. An acidizing composition, accordingly, must first of all be reactive with the formation to convert at least portions thereof into products which are soluble to some extent, The formation commonly acidized is composed, at least in part, of CaCO The additament which is reactive with the formation is usually an acid, hydrochloric acid commonly being employed, and containing a small amount of a corrosion inhibitor to protect metal parts against acid corrosion. It is also highly desirable that the rate of reaction of a composition used in acidizing formations traversed by a Well be controllable as, for example, that the rate of reaction be suitably retarded so that the acidizing composition does not spend itself only upon portions of the formation immediately adjacent to the borehole, perhaps to the extent of creating undesirable cavities there, and failing to penetrate the formation to an appreciable distance from the borehole. Such uncontrolled attack on the formation does not permit extension of the acidizing treatment suificiently far from the wellbore to be considered satisfactory.

Aqueous media used in such Well treating operations as fracturing, acidizing, and the like, must be resistant to excessive loss thereof to the formation. Such loss leads to poor results and high cost.

Uncontrolled attack of an acidizing solution on the rock in the formation with which it first comes in contact results in little or no attack on rock further removed from the wellbore; this is highly undesirable.

A need exists for a more effective method of reworking wells including fracturing and acidizing, particularly when acidizing is combined with fracturing in a single operation. There is a special need for a method employing an additament which is readily available, is economical to use, and can be conveniently admixed with an aqueous Workover or well treating composition to impart greater viscosity, less turbulence during pumping, less resistance to flow, lower fluid loss to the formation, and a retarded controlled attack on calcium carbonate-containing formations to Workover and treating compositions.

The principal object of the invention is to provide such improved method wherein wells may be reworked and treated more elliciently and more effectively.

The preparation and the properties of the composition of the invention and the method of use thereof in oil well Workover and treating operations will be made clear in the ensuing description and are specifically defined in the appendant claims.

The invention, accordingly, is a method of reworking and treating a well which encompasses and contemplates repairing and cleaning wells, positioning packers, perforating casing, cementing well casing, sealing off connate water, hydraulic fracturing, acidizing, dem-ulsifying oil and water emulsions, flushing debris from the well, and the like.

The composition employed in the method of the invention comprises water or brine having admixed therewith between 1 pound and 200 pounds, per thousand gallons of the composition, of an anionic polysaccharide known as Polysaccharide B1409, hereinafter usually referred to merely as B1459.

13-1459 is procurable from the United States Department of Agriculture, Agricultural Research Service, Northern Utilization Research and Development Division, Peoria, Illinois or by contacting the United States Department of Agriculture, Washington 25, D.C.

B-1459 is considered to have the molar ratio of mannose, glucose, glucuronate, and acetyl groups of 3:3:2:2, respectively, as determined by recent tests by the United States Department of Agriculture, although earlier work estimated the molar ratio of the mannose, glucose, glucuronate, and acetyl groups to be 2:2:1:1, respectively as described in the publication identified as CAN9 of September, 1959, entitled Information on Polysaccharide B-1459, published by the United States Department of Agriculture, Agricultural Research Service, Northern Utilization Research and Development Division, Peoria, Illinois.

B-1459 is normally a soft, bulky, slightly colored powder, having a molucular weight of about 1 X10 and being anionic in nature. It has high resistance to bacterial action and remarkable stability, particularly in the presence of a polyvalent metal salt such as calcium chloride. It may be dissolved in water at room temperature or a higher temperature to produce a non-thixotropic fluid which shows a tendency to gelation. It has a pH value in the range of from about 7 to about 8.5 when in an aqueous concentration of from 0.1 to 1.0% by weight. It shows substantially no rotation of polarized light.

B-1459 may be prepared in water solution containing between 2.5 and 3 percent dextrose, 0.4 percent of a nitrogen source known as distiller solubles, procurable from the major distilling industries by such identification, about 0.5 percent dipotassium phosphate, and about 0.01 percent Epsom salts, to which is admixed an inoculating bacterium known as Xanthornonas Campestris; a fermentation period thereafter provided. The composition so prepared preferably has a potassium content of not more than 5.4 percent by weight and a chloride content sufficiently low to represent less than 0.3 percent KCl. The nitrogen content is preferably less than about 0.4 percent by weight and a phosphorus content of not more than 0.2 percent by weight. It usually has a specific rotation of about zero.

In the preparation of B-1459 the pH of the solution is is adjusted to 7. The solution is sterilized by heating to about 275 F. for about three minutes. The solution is then cooled by passing it through a heat exchanger until a temperature of between about 80 and 84 F. is obtained. The solution is then inoculated by adding a culture of the bacterium Xanthomonas Campestris in an amount of about percent, by volume of the solution, to make a broth. The pH is maintained thereafter, by the addition of the dipotassium phosphate, at a value of between 6.2 and 6.7. Following inoculation with the bacterium, the broth is allowed to ferment for a total of about 96 hours. During this period the broth is agitated and aerated. Aeration is usually provided by admitting a stream of air at the bottom of the fermentation bath. After about 24 hours of the 96-hour period, a noticeable increase in viscosity is observed which continues to increase until a final viscosity of about 7000 centipoises (measured by the Brookfield viscosimeter) is reached at the end of 96 hours. The broth is then drumdried. It

contains about 50 percent of the active polysaccharide above described. The remaining 50 percent consists mainly of soluble sugars and salts plus some insoluble cell walls which have no deleterious effect on the composition for use in the practice of the invention.

Additional information on the nature and properties of Polysaccharide B-1459 may be obtained by referring to the aforementioned publication CA-N-9 of the United States Department of Agriculture or the Journal of Applied Polyscience, volume V, issue 7, pages 519-526 (1961) entitled Polysaccharide B1459: A New Hydrocolloid Polyelectrolyte produced from Glucose from Bacteria Fermentation by Jeanes, Pittsley, and Senti of the United States Department of Agriculture.

Additional information may also be obtained from licensed manufacturers of Polysaccharide B4459, e.g., Archer-Daniels-Midland Company, 733 Marquette Avenue, Minneapolis, Minnesota, which has such information available under ADM Technical Data Publication, e.g., that identified as No. B-8, entitled ADM Experimental Product 7097, dated July 18, 1962.

Because of its stability in the presence of salts, it is particularly effective and highly desirable as a fluid loss agent and retardant to CaCO attack by acids in well workover and well treating operations employing a brine, or where brine may be encountered during the treatment.

In the presence of appreciable amounts of dissolved salts in the water, e.g., brines having a specific gravity of about 1.05 or more, i.e., the so-called heavy brines, a less amount of the polysaccharide need be employed than in light brines, i.e., those having a specific gravity of between 1 and 1.05 or in substantially fresh water to attain comparable beneficial effects on viscosity and fluid loss. Usually there are employed, in fresh water, between about 48 and 100 pounds or more of the abovedescribed polysaccharide per 1000 gallons of the composition whereas in light brines, there are employed, between about 12 and 100 pounds thereof and in heavy brines, there are employed, between about 2 and 100 pounds of the polysaccharide per 1000 gallons of the resulting composition.

A series of tests (designated Series One) was run to show the effect of adding the B-l459, as above described, to aqueous well workover compositions. The effectiveness of the additive was tried in brines, designated A to E, having the following characteristics:

Brine A, comprising 22 percent by weight CaCl and 78 percent water and having a density of ten pounds per gallon.

Brine B, comprising 33 percent by weight CaCl and 67 percent water and having a density of 11 pounds per gallon.

Brine C, comprising 20 percent by weight CaCI 33 percent ZnCl and 47 percent Water and having a density of 14 pounds per gallon.

Brine D, (to simulate a typical natural brine) comprising 11.5 percent by weight NaCl, 2.1 CaCl 1.5 percent MgCl 6H O, and 84.9 percent water and having a density of 10 pounds per gallon.

Brine E, comprising 7.5 percent NaCl and 92.5 percent water and having a density of 8.79 pounds per gallon.

The fluid loss of both fresh water and the abovedescribed brines, with or without the weight of the polysaccharide set out in the following table per thousand gallons of the treating fluid, was measured. The fluid loss was determined by employing the apparatus described in API RP 29 employing either 1) a filtration area of 7.07 square inches, a pressure cell having a fluid capacity of 300 milliliters, and carrying out the test at 80 F. and p.s.i.g. pressure, or (2) a filtration area of 3.72 square inches and a pressure cell having a fluid capacity of milliliters and at a temperature of F. and 1000 p.s.i.g.

pressure, according to the designation set forth by the table.

TABLE I [Eflect of polysaccharide B-1450 on fluid loss of fresh water and designated brines] Polysaceharide Fluid loss in milliliters (m1.) after stated time (Time in minutes) Test Aqueous B-1459 in Type of fluid solution pounds per loss apparatus 1,000 gallons 1 4 9 16 25 1 Fresh water..- (1) 300 2 do (2) 160 129 142 146 150 154 300 in 2 min. and 10 sec. 1 278 in 25 min. 2 18 28 41 54 77 4 17 24 32 42 53 6 6. 2 10. 2 15. 8 22. 2 30. 2 12 4. 3 6.8 9.8 13. 2 16. 8 48 0. 8 2. 6 4. 8 7. 9. 3 48 21. 8 23. 4 25. 2 26.8 28. 0 48 0 3. 4 7. 2 10. 8 13. 6

300 in 1 min. and 49 see. 6 6 9. 13. 5 18 23 12 21 24 27. 4 31 34. 8 12 13. 8 20. 4 25. 6 31. 6 12 5. 5 8.2 11. 4 14. 8 18.4 12 15. 4 20.8 25. 6 30. 4 35. 6 48 5. 1 7. 8 11 14. 2 17. 6

(1) Fluid loss apparatus, described in API RP 29 for low temperature, low pressure work: filtration area=7.07 infl; fluid capacity of test cel1=300 ml.; tests made at 80 F. and 100 p.s.i.

(2) Fluid loss apparatus for high temperature, high pressure work: filtration area=3.72 in}; fluid capacity of test cell=described in API RP 29,

i 100 1111.; test made at 180 F. and 1,000 p.s.1.g.

(3) Fluid loss apparatus similar to (2) except test made at 200 F. and 1,500 p.s.i.g.

Portions of the brine and B-1459 compositions of variations in the amount of B-1459 added to a brine of Series One as tested and the results reported in Table I the type employed in workover fluids. The brine emabove, were allowed to stand for 14 days at the advanced ployed was Brine A, described above. The amounts of temperatures of either 250 F. or 275 F. The thus aged 39 B4459 are set out in Table III below. The procedure compositions were tested for fluid loss properties and consisted in adding B-145'9 accompanied by agitation. found to show but a slight increase in fluid loss when The viscosity values obtained are also set out in Table III. tested after the 14-day period, as compared to the tests TABLE HI obtained on the substantially freshly prepared compositions. 35 [Viscosity at 80 F. after 1 hour] Reference to Table I shows that the fluid loss, as measured by the standard API filtration tests employed, Test Pounds of B1450 per Viscosity in was very definitely lessened for the fresh water and each 1,000 ga1111$ Bum-A centipoises of the brines employed.

A second series of tests was run to show the effect of g; E8 the presence of B4459, when employed in the practice 39:: 2 39 of the invention, on viscosity of aqueous fluids employed 40 1 20 in reworking wells. The tests were conducted similarly 10 those set out in Table I, ViZ., by merely admixing r Viscosity measurements were made usinga-Brookfield Synchro-lectrio B4459 in the amount set out in Table II with fresh M0591 LVF Vlscoslmetel water or the brine there indicated. Reference to Table III shows that substantial improve- TABLE II [Viscosity of various aqueous solutions containing Polysaccharide B-1459] 13-1459 in Viscosity in c.p.s. after various times in hours Test Aqueous pounds per Temp.

solution 1,000 gallons in F.

21 Freshwater.-. 80 1 3 1,340 so 1, 600 6, 566 1, 400 5,800 3 2, 900 40, 000 40, 000 7, 600 200 70 58 11s 74 10,500 6,200 1, 110 330 200 100 12s cps. after 18 days so 25 so 290 290 380 300 so 5,000 7, 600 7, 600 3, see 200 58 2s 92 so 200 226 110 92 Brine D 80 200 Viscosity measurements made using Brookfield Synchro-lectric Viseosimeter, Model LVF.

Reference to Table II shows that the viscosity was inments in viscosity result when the B4459 is added to creased in the fresh water and very markedly increased a CaCl brine in accordance with the invention. It is in the brines employed. Such improvement in viscosity recommended that at least 2 pounds of B-l459 per 1000 is of great importance both in rendering the liquid more gallons of brine be employed although some benefit effective in suspending materials contained therein and resulted from the use of 1 pound of the polysaccharide. in a reduction in turbulence when pumped, which is of A fourth series of tests was run to show the effect, value in moving such fluids through pipes and tubing. if any, of the presence of Polysaccharide B-14S9, on the A third series of tests was run to show the effect of permanent permeability of a formation to flow from the TABLE IV Concentration of B- Percent damage to Test 1459 in lb./1,000 gal. of formation as calculated No. brine in par. 623 of section VI, API RP 39 41"" None 49. 42 24 51. 0 43 48 53. 0

1 In tests 42 and 43, the 2-inch diameter, 1-inch long Berea sandstone cores (required by the test) were first saturated with 3% by weight aqueous CaCl solution.

Reference to Table IV shows that the composition employed in the method of the invention does not result in any significant plugging of a formation when back flowed after treatment with the brine and B-1459 composition.

A fifth series of tests was run to show that the separate ingredients employed in the preparation of Polysaccharide B1459, mere physical mixtures thereof, or other selected polysaccharides are not satisfactory or acceptable in the practice of the invention. The tests were made employing Brine A, as above described, at 80 F. The separate ingredients, the physical admixture of the individual components which when chemically united go to make up the Polysaccharide B1459, or other polysaccharides, viz., xylan and chitin, were admixed with the brine and the viscosity tested, as in the runs set out in the preceding tables. The results obtained in these runs are set out in Table V.

TABLE V [Eilect of various polysaccharide components on viscosity of brine temperature employed was 80 F.]

Test No. Concentration additive per 1bs./-

1,000 gallons of Brine A Viscosity 1 p None 48 D-rnannose.

48 Acetyl acetone 2 48 313:2:2 mixture of D-mannose: glucose:potassiurn glucuronate and acetyl acetone, respectively.

la 16 48 Chitin- 12 Polysaccharide 13-1459 720 51 37 (Table III) 3 1 Viscosity measurements made using Brookfield Synchro-lcctric Viscosimeter Model LVF.

Z The acetyl acetone was added in an attempt to provide active acetyl groups as the active ingredient.

3 Test 37 is repeated here from Table III for convenient reference.

Example 7800 pounds of a water are placed in a suitable tank, pit, or the like, provided with appropriate stirring equipment. 2200 pounds of CaC-l are admixed with the water to produce 1000 gallons of a brine having a density of about 10 pounds per gallon. (Such density is satisfactory for a workover fluid for reperforating and the like but CaOl brine, unmodified, is not sufliciently viscous for such use since its viscosity is only about 7 cps. Furthermore, the fluid loss, as determined by the above-described API filtration test, is 300 milliters in 2 minutes and 10 seconds, (see Run 4, Table I) which is far too great for use of the brine, unmodified, as a workover fluid.)

In accordance with the practice of the invention, 24 pounds of B-1459, as above described, is admixed with each 1000 gallons of the brine accompanied by vigorous stirring. The resulting composition has a viscosity of 1600 cps. at F. and a fluid loss, by the high pressurehigh temperature API filtration method (1000 p.s.i. at 180 F.), of 15 milliliters in 25 minutes. The composition so made is then injected down the cased wellbore and positioned (spotted) in the casing at the level where it is desired to" perforate the casing. Ordinary drilling mud or other liquid to provide hydraulic pressure may be injected down on top of the perforating fluid. A perforating gum is then lowered down the wellbore through the fluids in the casing and the casing then perforated as desired.

The high density clear workover fluid containing B- 1459 provides the desired pressure in the casing, is resistant to deterioration by heat, is sufliciently viscous to maintain a definite phase from the super-positioned mud, and is non-contaminating of the formation.

After perforation is accomplished, the gum is withdrawn from the well and the mud, followed by an appreciable portion of the clear workover fluid are flushed from the well.

Although this embodiment of the method of the invention may be practiced in substantially the same manner when employing either brines or fresh water, it is recommended (as aforesaid) that the amount of additive employed in water be somewhat greater. For example, whereas as little as 1 pound of the B-1459 may be sufficient per 1000 gallons of the composition employing heavy brines, i.e., brines of greater specific gravity than 1.05, between 12 and pounds of B-1459 are recommended per 1000 gallons of composition employing light brines, i.e., those having a specific gravity of more than 1.01 and less than 1.05, and at least about 48 pounds, per 1000 gallons of composition, are recommended for compositions employing substantially fresh water as the aqueous medium.

The method of the invention is especially adapted to acidizing CaCO -containing formations penetrated by a well. According to this embodiment of the invention, B-1459 is incorporated or admixed with an aqueous acid or an oil-water or brine emulsion containing some acid, within the l to 100 pounds thereof per 1000 gallons of acidizing composition, similarly as when drilling or reworking a well. The acidizing composition is injected down a wellbore and back into the formation, usually by pumping under pressure. The acid composition thus injected attacks the formation and converts portions thereof to soluble materials, e.g., CaCO CO and water. The presence of the B-1459' desirably retards the acid attack on the CaCO rock as well as lessens the loss of fluid undesirable to the unconsolidated portions of the formation.

In more detail, this embodiment of the invention is carried out by admixing a prepared aqueous acidic solution, e.g., one of HCl, formic, or acetic, from 1% by weight to the point of saturation, and B-1459, either dry or in an aqueous solution, in a suitable mixer, and injecting the resulting substantially uniformly mixed aqueous composition down a wellbore previously provided with suitably positioned packers so that the treating composition can be located or spotted at the desired elevation or stratum to be acidized. It may be found convenient to add the B-1459 to the acid solution as it enters the well, taking advantage of the turbulence to enhance mixing. Suiticient pressure is applied during injection to force the acidizing composition containing the retardant, fluid loss inhibitor B-1459, back into the formation and into any existing fractures present. According to one embodiment of the invention, sufiicient pressure is applied during injection of the aqueous composition to create fractures in the formation and thus attain the benefits of both fracturing and acidizing in one operation.

A number of additional series of test runs was made which showed the efficacy of the invention in contrast to acidizing according to conventional practice. The tests were carried out by exposing weighed samples of a calcium carbonate rock, of specific size, to a measured amount of an aqueous acid solution of known strength, confined in a stainless steel reaction chamber under a high nitrogen gas pressure to simulate the pressures in subterranean formations. The time of reaction between the acid and the calcium carbonate rock was carefully timed. Agitation was provided during the reaction period to approximate the turbulence existing during the injection operation. At the end of the test runs, the pressure was released and the sample of calcium carbonate rock remaining was removed and washed wtih water, dried, and reweighed. The loss of weight of the sample during the test was then ascertained and such weight loss divided by the exposed surface area of the calcium carbonate rock sample to give the pounds of rock removed, by attack of acid per square foot of area per second of reaction.

SERIES SIX The test conditions which were common to all of the tests of this series are as follows:

Type of CaCO rock r Alabama cream marble. Temperature in F. 80. Test time in minutes 2. Pressure under N 1000 p.s.i.g. (gauge). Size of rock samples 1" x 1" x 0.3

(2.3 in. Acid employed by weight Ratio of surface of samples in in. to aqueous HCl.

volume ratio of acid in in. 1: 1.

The tests were carried out by placing each weighed sample of marble in a compartment and the aqueous acid solution in another compartment of the same steel vessel so constructed that the acid remained separate from the marble so long as the vessel remained upright. A gas inlet, positioned in the reaction vessel, was connected to a nitrogen gas source. The time was observed and the reaction vessel containing the aqueous acidic composition and marble in separate compartments Was inverted to bring the acidic solution and marble into contact. The reaction vessel was placed in a constant temperature bath of 130 C. and also placed on a mechanical shaker which provided 130 oscillations per minute through a 1.5- to 2-inch oscillation.

After the expiration of the desired time, the steel reaction vessel (and the compartments therein) were rein- 10 tests are shown in Table VI. Comparative Tests A, B, and C were made to show the marked difference in the results obtained when either no B-l459 was employed or other gums which are not equivalent to the B-1459.

TABLE VI Percent by wt. of additive Reaction rate in 0.36 Karaya gum 0.72 Guar gum By reference to Table VI, it can be seen that B-1459 reduced the rate of reaction between the acid and the calcium carbonate rock when used in as small an amount as 0.03%, based on the weight of the aqueous acidizing composition, and that such retarding effect continues to decrease as the amount of the B-1459 is increased to as much as 1.80% by weight of the acidizing composition. It is also shown in the table that the use of either karaya gum or guar gum by no means attains the retardation eifect attained by the use of the 18-1459. Continued lessening of the rate of reaction beyond 1.8%, by increasing the percent of the B-l459 employed, is apparent by extrapolation of the results shown in the table. However, it must be borne in mind that the retardation of the acidizing action must not be so great that insuflicient acid attack on the rock results. The amount of the B-l459 used may be between about 0.03 and 1.8%, but is preferably between about 0.12 and 1.08%, by weight of the aqueous acidizing composition.

SERIES SEVEN Since the acidizing composition, employed during acidizing a formation, is progressing or advancing into the formation or, in other words, is in a state of motion (particularly in the earlier part of the operation) the reaction rates of acid solutions with or without the benefit of the presence of B-1459 were evaluated for varying flow conditions in a series of tests. In carrying out this series of tests, acid solutions of known concentration were caused to flow through prepared samples of calcium carbonate rock. The samples were prepared by cutting pieces of CaCO rock (marble being employed about 9.5 inches measured along the bedding planes) along a plane to provide two parts with mating faces and then repositioning the parts substantially as they were originally in the piece of rock, but spaced apart 0.1 inch to provide a simulated fracture. Each so made sample had a simulated 0.1 inch fracture which was 1.5 inches wide and extended 9.5 inches along a bedding plane of the piece of rock. The rock samples were weighed before and after each test and the weight loss in lb./ft. sec. was calculated from the weight of sample dissolved, the area of rock exposed along the simulated fracture, and the time of reaction. In each of Examples 61 to 66 of the invention, and in Comparative Runs D to H, set out in Table VII below, the rate of flow was calculated from the volume flow rate and the cross-sectional area of the simulated fracture. The reaction rate was calculated by determining the calcium content of the efiluent acid, and the sample area, and the reaction time.

By reference to Table VII, it can be seen that the increased velocity or rate of flow in all instances increased the reaction rate. It is clearly shown, however, in the table, that the reaction rate when accelerated by the in creased rate of flow, when employing either 0.12 or 0.36% 13-1459 (based on the Weight of the aqueous acidizing composition), Was markedly retarded in contrast to the rate of reaction at the higher rates of flow when no B1459 was present. By comparing the reaction rate of the acidic composition containing the B-1459 with the acidic composition containing karaya gum or guar gum, in comparable amounts of additive, at a flow rate of 2 feet per second, it is quite apparent that the retarding effect brought about by the karaya and guar gum was definitely less than that shown by comparable amounts of B-1459.

SERIES EIGHT Another series of tests was run to show the effect that treatment of a calcium carbonate-containing rock with an acidizing composition containing 13-1459 has upon a subsequent treatment in reducing the reactivity during such subsequent treatment. These tests are designated Examples 67 and 68. Example 67 was conducted by treating a weighed marble specimen, of the size described in Series One above, with a 15% by weight aqueous HCl solution containing 0.36% by weight of B-1459 for 1 minute. This is designated 67a on Table VIII. The specimen was then treated with 15% by weight aqueous HCl and again Washed, rinsed, and reweighed. The results are designated 67!) in Table VIII. Example 68 was conducted by treating a weighed specimen as in Example 67, for 1 minute in 15% by weight aqueous HCl solution, designated 68a but, thereafter, without washing or drying, immediately treating the specimen with 15% aqueous HCl solution. The results are shown in Table VIII as 6817.

The purpose of the eighth series of tests was to show the persistency of the retardation effect produced by the B-1459 on subsequent acid treatments in which there was no retardant present. The conditions common to each of the tests in this series were the same as those employed in the first series hereinabove.

TABLE VIII Weight loss in grams Test time in min.

Percent by wt. B4459 Test Explanation Sample so treated washed, dried, and weighed.

Dried sample of 67a treated with aqueous 1101 only (no 13-1459) and again washed, dried, and rewcighed.

671) None 1 1. 1029 Total 2 1. 6625 6821 0.36 1 Same as sample 67a,but

was NOT washed, dried, nor weighed but removed and treated with aqueous 1101 only.

Sample 68a, washed,

68b None 1 dried, and rewoighed.

Total 2 1. 4544 Reference to Example -67 of Table VIII shows that when CaCO -containing rock is treated with aqueous HCl acid containing 13-1459 and the treated surface is not washed to remove any B1459, the rate of reaction when subsequently exposed to aqueous I-ICl acid containing no B-l459 is slower than the rate of reaction on a sample which was untreated or which had the adhering acid containing B1459 washed off before the treatment with HCl containing no B1459. In other words, the presence of Bl459 in an acid solution slows down subsequent acid attack, apparently providing a protective film thereon.

SERIES NINE TABLE IX Acid Reaction rate in Test B-1450 in percent by wt.

J 24% acetic one Reference to Table IX shows that the rate of reaction of either formic or acetic acid on CaCO rock is definitely retarded by the presence of B4459. The presence of B-1459, in addition to its desirable retarding effects on the rate of reaction of an aqueous acid solution on CaCO rock, also exhibits a marked beneficial reduction in fluid loss to a formation during acidizing and fracturing. A series of tests, identified as Series Ten and was run as follows:

SERIES TEN Bedford limestone, having a permeability of 0.3 millidarcie to air, was cut into cores, 1 inch in diameter and 3 inches long. The fluid loss was ascertained according to Section V of API RP 39 1st Edition (July 1960). The temperature employed was F. and the pressure 1000 p.s.i.g. The fluid employed was a 15 by weight aqueous solution of I-ICl containing 0. 4% by weight of an amine type inhibitor against metal corrosion, and in the tests so indicated, up to 300 pounds of CaCl per 1000 gallons.

Compositions prepared were also ascertained on a Fann viscosimeter at 80 F. and at rpm. The results are shown in Table X.

TABLE X Volume in mi. through 13-1459 in CaCl; in core at end of indicated Viscosity in centi- Test No. 1b./1,000 lb./1,000 minutes poises measured on gallons gallons Fann Viscosirneter,

80 F., 100 r.p.m. 1 4 9 16 25 30 0.2 (All fluid in cell 18.0

through in 1.3 min.)

Reference to Table X shows that the presence of the 13-1459 greatly lessens the fluid loss according to the standard test of Section V of API RP 39. It shows that it is particularly eifective in brines.

When guar gum or karaya gum was employed in 15% aqueous HCl, in the amount of 40 pounds of the gum per 1000 gallons of the aqueous HCl, and the viscosity on the Fann viscosimeter ascertained at 80 F. and 100 r.p.m., the highest viscosity value obtained after 1 hour standing was 3.4 centipoises and the highest viscosity obtained after 6 hours standing was 8.4 centipoises. Longer standing thereafter showed a gradual decline in viscosity down to 2.7 centipoises after 24 hours standing.

The practice of the invention offers a number of advantages in reworking a well in its broad sense. Examples set out in Tables VI to X show that when either fracturing or acidizing a formation, particularly when employing a brine; there is a retardation of acid attack on formation rock containing CaCO there is increased viscosity and suspending properties for sand employed as a propping agent in fracturing; there is a marked reduction in fluid loss to the formation in both acidizing and fracturing; the composition employed in the invention is highly resistant to deterioration due to heat, even at temperatures approaching 300 R, which are in excess of this customarily encountered in fluid-producing formation,

Having described my invention, what I claim and desire to protect by Letters Patent is:

1. A method of acidizing a calcareous subterranean formation traversed by a well consisting of admixing, with an aqueous acidic fluid selected from the class consisting of solutions of (1) water and acid and (2) brine and acid, a polysaccharide comprising combined groups of: mannose, glucose, potassium glucuronate, and ace-tyl groups in the molar ratio of about 3:3:2:2, which has a potassium content of about 5.4% of which no more than 0.3% is derived from potassium chloride, contains not over about 0.4% nitrogen and not over about 0.2% phosphorus, which has a specific rotation of about zero, has a pH value of between about 7 and about 8. 5 in an aqueous concentration of from about 0.1 to 1.0%, and has a molecular weight of at least about 1x10 to make an acidizing composition of decreased fluid loss, of retarded attack of calcareous rock, and of increased resistance to deterioration by heat; injecting the acidizing composition so made down the wellbore of the well at sufficient pressure to force at least a portion thereof into the formation into contact with the exposed faces of the formation to effectuate a controlled attack by the acid on the calcareous formation.

2. The method according to claim 1 wherein the polysaccharide is Polysaccharide B-1459.

3. The method according to claim 1 wherein the polysaccharide is employed in an amount between about 0.03 and about 1.80% by weight based on the weight of the resulting treating composition so made.

4. The method according to claim 1 wherein the aqueous acidic fluid is a solution of brine and acid.

References Cited by the Examiner UNITED STATES PATENTS 2,596,137 5/1952 Fast 252-8.55 2,689,230 9/ 1954 Cardwell et al l6642 2,885,004 5/ 1959 Perry l6642 3,000,790 9/ 1961 Jeanes et al -3l 3,007,8 79 11/1961 Jordon 252-8.5 3,020,207 2/ 1962 Patton 195-31 OTHER REFERENCES US. Department of Agriculture Publication CA-N-9 Information of Polysaccharide B 1459, September 1959, S, 584,U5a4.

CHARLES E. OCONNELL, Primary Examiner. 

1. A METHOD OF ACIDIZING A CALCAREOUS SUBTERRANEAN FORMATION TRAVERSED BY A WELL CONSISTING OF ADMIXING, WITH AN AQUEOUS ACIDIC FLUID SELECTED FROM THE CLASS CONSISTING OF SOLUTIONS OF (1) WATER AND ACID AND (2) BRINE AND ACID, A POLYSACCHARIDE COMPRISING COMBINED GROUPS OF: MANNOSE, GLUCOSE, POTASSIUM GLUCURONATE, AND ACETYL GROUPS IN THE MOLAR RATIO OF ABOUT 3:3:2:2, WHICH HAS A POTASSIUM CONTENT OF ABOUT 5.4% OF WHICH NO MORE THAN 0.3% IS DERIVED FROM POTASSIUM CHLORIDE, CONTAINS NOT OVER ABOUT 0.4% NITROGEN AND NOT OVER ABOUT 0.2% PHOSPHORUS, WHICH HAS A SPECIFIC ROTATION OF ABOUT ZERO, HAS A PH VALUE OF BETWEEN ABOUT 7 AND ABOUT 8.5 IN AN AQUEOUS CONCENTRATION OF FROM ABOUT 0.1 TO 1.0%, AND HAS A MOLECULAR WEIGHT OF AT LEAST ABOUT 1 X 10**6, TO MAKE AN ACIDIZING COMPOSITION OF DECREASED FLUID LOSS, OF RETARDED ATTACK OF CALCAREOUS ROCK, AND OF INCREASED RESISTANCE TO DETERIORATION BY HEAT; INJECTING THE ACIDIZING COMPOSITION SO MADE DOWN THE WELLBORE OF THE WELL AT SUFFICIENT PRESSURE TO FORCE AT LEAST A PORTION THEREOF INTO THE FORMATION INTO CONTACT WITH THE EXPOSED FACES OF THE FORMATION TO EFFECTUATE A CONTROLLED ATTACK BY THE ACID ON THE CALCAREOUS FORMATION. 